My personal approach is to slow cook chicken breast in whole in water, with light seasoning (some salt and minimum spices), then use the broth to cook the sauce and vegetable.
Then shred the chicken and mix the two and keep warm for a while to let the flavors soak.
The reason for this is:
1) The curry flavor doesn't penetrate the chicken much at all, so for the chicken to carry flavors it has to be shredded and fully soaked with the sauce. Cooking chicken in the sauce doesn't help with flavor much based on my experience. A thick sauce and large surface area of the chicken to absorb it is a much more effective flavor delivery mechanism.
2) There is a conflicting requirement for temperature for cooking chicken and the sauce. The chicken would need a lower temperature much below boiling temperature, while the onion, tomato, and other vegetable need much higher temperature to "melt" into the body of the sauce.
3) The choice of simmering chicken in water is more due to energy efficiency and convenience (with an electric slow cooker). Also when you keep the meat a whole piece you don't lose much juice at all, compared with other "dry" cooking method. Also since the broth goes into the final product anyway, you are not wasting anything.
4) Shredded chicken has a better texture than cut chicken.
One catch is, since all the broth goes into the product, then so does all the salt and water. You need to measure those carefully up front.